(a) Field of the Invention
The present invention concerns a method of fabricating an amorphous silicon film which is employed in electrophotography to serve as a photoreceptor in copying machines, and more particularly it pertains to a method of fabricating an amorphous silicon film having a high-resistivity characteristic in dark conditions.
(b) Description of the Prior Art
Amorphous silicon (hereinafter to be referred to briefly as a-Si) has, of late, been attracting the interest of those who are in the technical fields of semiconductor devices as having a great potential to provide a semiconductor material employed in obtaining, at low cost, semiconductor devices having a large working area, because of the advantages such that the semiconducting, insulating and photoconducting characteristics of the a-Si film can be controlled at will by designing its production method and by controlling the conditions with which an impurity is doped therein. Also, in view of the fact that the solid condition of the a-Si film is amorphous, there is no need to go so far as to consider lattic regularity or lattic mismatch relative to a semiconductor substrate as has been required to take into consideration when a single crystal is used.
Especially in recent years, researches have been being made for the employment of a-Si films to serve as photoreceptors for electrophotography copying machines in view of the reasons that a-Si, as compared with such substances as amorphous selenium and Cds, has a greater hardness and is superior in its heat-resistant and chemically resistant properties.
Among those various characteristics which are required of those semiconductor materials which are employed to form a photoreceptor for electrophotography, there are two which are especially important. One is the highly resistive characteristic when the material is placed in dark conditions, and the other is represented by its photoconductive characteristic when it is placed in the conditions of being illuminated by light rays.
The greatest problem which has been encountered in placing a-Si films to practice is that it has been quite difficult to obtain said characteristic of being highly resistive in darkness.
According to the electrophotography which has been invented by C. F. Carlson, the initial step of copying a character or an image is to load, through corona discharge in darkness, an electric charge onto the surface of a semiconductor material serving as a photoreceptor, whereby the surface potential of the semiconductor material is elevated (which, however, is lowered in case of a negative corona discharge) through this charge-up. It is required that the holding of the charge will last for a lengthy period of time when placed in darkness. In other words, the attenuation of the surface potential is required to take a lengthy period of time. To this end, the semiconductor material employed is required to have as high a resistivity as being close to that of an insulator.
In order to use this a-Si film to serve as a photoreceptor, attempts have been made to form an a-Si film on a metal substrate such as aluminum, but there has not been obtained a satisfactorily high resistivity therefrom. Since an a-Si film is a structure-sensitive material, it should be understood that, in case an a-Si film is formed without any doping of an impurity, it often happens that the resulting film is of an n type with a low resistivity, which is considered to be due to various defects produced within the film. Even when a Group III impurity is doped at the time of forming a film for the purpose of compensating for n-type carriers, the resulting resistivity will be at the degree of only 10.sup.10 -10.sup.12 .OMEGA.-cm at the highest. With such values of resistivity, the length of time of holding the charge is noted to be too short to be used for practical applications in copying machines of electrophotography. As a means of solving such a drawback of the prior art a-Si films, there has been proposed a method of doping, at the time an a-Si film is formed, an impurity or impurities such as N.sub.2 and O.sub.2, or O.sub.2 and B, or O.sub. 2 plus B and P, or there has been proposed by the present inventors in their pending U.S. Ser. No. 342,650, a method of doping impurities N.sub.2 and B, or N.sub.2 plus B and P, to obtain an a-Si film having a high resistivity of a level above 10.sup.13 .OMEGA.-cm.
However, in case such prior methods are carried out, the following problems arise. As discussed above, an a-Si film which serves as a photoreceptor requires, under the condition of being illuminated by light rays, to be superior in its photoconducting characteristic as a photoreceptor. In order to improve this photoconductivity, it should be understood that, since carriers are to be developed through illumination by light rays, the a-Si film which is produced is required to be able to use light rays of as great a wavelength as possible, i.e. the wavelength up to the absorption edge must be great, and the lifetime of the a-Si film is required to be long, and further the mobility of carriers is required to be large. In case a p type impurity gas as stated above or an N.sub.2 or O.sub.2 impurity gas is introduced, the a-Si film which is produced will be obtained as a highly resistive material when placed in darkness. However, there are formed, within the a-Si film, a number of energy levels due to such impurity or impurities as mentiond above, and carriers will be either trapped or scattered thereby, leading to a reduced mobility of carriers and to a shortened lifetime of the a-Si film, so that its photoconductivity will no longer become sensitive.
Even when a high resistive a-Si film is desired by relying on N atoms or O atoms as a result of the introduction of N.sub.2 gas or O.sub.2 gas at the time of production of the a-Si film, it should be noted that, in case the amount of such gas which is introduced is too small, no sufficiently high resistivity can be obtained, whereas in case the amount of the dose of such impurity gas or gases is or are too large, there can be obtained a high resistivity, but the photosensitivity of the a-Si film will drop remarkably. In an extreme case, the a-Si film can become so highly resistive as to be an insulator. Thus, controlling of the amount of the dosage of impurities has been very difficult to control in the prior art.